The invention relates to a transparent substrate, especially made of glass, intended to be incorporated into glazing and provided, on at least one of its faces, with an antireflection coating.
An antireflection coating usually consists of a stack of interferential thin layers, generally an alternation of layers based on a dielectric material having high and low refractive indices. The purpose of such a coating, deposited on a transparent substrate, is to reduce its light reflection, and therefore to increase its light transmission. A substrate coated in this way therefore has its transmitted light/reflected light ratio increased, thereby improving the visibility of objects placed behind it. When it is desired to achieve the maximum antireflection effect it is then preferable to provide both faces of the substrate with this type of coating.
One useful application is in the equipping of buildings. Glazing with an antireflection effect is, according to this application, used, for example, as shop windows so as to better distinguish objects placed behind the window, even in configurations in which the interior lighting is low compared with that outside. It may also be used as counter glass.
Recently, another application has been envisaged, especially at the request of motor-vehicle manufacturers. This is to give a windscreen an antireflection effect while still meeting the requirements in force which insist on high levels of light transmission, generally greater than 75%, and a very low residual haze (less than 1% of the transmitted light). The desired antireflection effect may, for example, consist in increasing the light transmission TL, which amounts to improving the visual comfort of the driver and his passengers. This also reduces the undesirable reflections which may hinder the driver, particularly the reflection of equipment inside the vehicle.
Hitherto, this application has not been developed to any great extent because of the fact that the level of mechanical and chemical durability required is difficult to achieve.
This is because the antireflection coating in the windscreen is placed at least on the 1 face, that is to say that face of the windscreen facing out of the passenger compartment.
However, that face of the windscreen is exposed to a number of stresses. Thus, the to-and-fro movement of the windscreen wipers when they are operating results in significant abrasion, and the impingement of various particlesxe2x80x94dust or chippingsxe2x80x94damages the coating in places, both from a mechanical standpoint and a chemical (corrosion) standpoint.
To solve these problems and therefore to develop glazing with an antireflection coating that has sufficient mechanical and chemical durability, for the purpose of being used as a windscreen, the Applicant has already proposed in document WO 97/43224 that at least some of the layers forming part of the antireflection stack of layers of materials having alternately high and low refractive indices be layers deposited by a pyrolysis technique.
This stack is not completely satisfactory since, because of the very fact that there is a large number of interfaces, the risk of the stack being fragile cannot be entirely avoided.
The object of the invention is therefore to propose an antireflection coating having improved mechanical performance, especially for the purpose of using the glazing which is provided with it as a motor-vehicle windscreen.
To do this, the subject of the invention is a transparent substrate, especially made of glass, provided on at least one of its faces with an antireflection coating A, characterized in that said coating consists of a single thin layer formed from at least two materials of different refractive indices, the composition of which thin layer varies continuously through its thickness. Preferably, the thin layer defined above has a geometrical thickness of between 100 and 400 nm.
Advantageously, this layer has at least one first region in its thickness (that region closest to the substrate) in which the refractive index increases and at least one second region (especially that closest to the atmosphere) in which the refractive index decreases. Between these two regions, there may also be an intermediate region of a relatively constant refractive index. Thus, it is possible to have a first region in which the index varies from the value I(0) of approximately 1.45 to 1.65 to the value I(1) of at least 2, especially 2.1 to 2.4, then an optional second region which is very thin compared with the first, in which the index remains approximately at the value I(2) and then a final region in which the index decreases again towards a value I(3), which may be closer to I(1) or may be slightly less than it. The particular feature of this layer is that the index changes gradually, so that this really acts as a monolayer. More complex index variations are also possible (more than two alternations of increasing index region and of decreasing index region).
Because of its actual construction, the antireflection coating according to the invention makes the glazing perfectly suited to use as a motor-vehicle windscreen.
Moreover, the mechanical integrity of the continuous layer allows it to undergo the curving operation without any difficulty.
Furthermore, the antireflection coating according to the invention has a very effective ultra-violet filtration effect, particularly in the wavelength range lying between 400 and 500 nanometers. This effect is advantageously combined with that of the thermoplastic, such as PVB, used as interlayer in the laminated glazing.
This thus avoids, even more effectively, the risk of the plastics and paints present inside the passenger compartment of the motor vehicle fading.
Advantageously, the layer is formed only from two materials of different refractive index.
According to this embodiment, the thin layer is preferably based on SiOxNy where x and y vary continuously through its thickness from 0 to 2 and from 0 to 1.33, respectively.
Still according to this embodiment, it is also preferable in that region of the layer which lies closest to the interface with the ambient environment, such as the air, for x to vary in a strictly increasing manner from 0 to 2 and for y to vary in a strictly decreasing manner from 1.33 to 0.
According to another embodiment, the thin layer according the invention is based on SizTi1-zO2 where z varies in a strictly continuous manner through its thickness from 0 to 1.
Preferably, in that reiogion of the layer which lies closest to the interface with the ambient environment, z varies in a strictly increasing manner from 0 to 1.
In order to optimize the antireflection effect of the graded-index xe2x80x9cmonolayerxe2x80x9d according to the invention, the reflective index of the thin layer preferably lies between 1.35 and 1.75, advantageously between 1.38 and 1.70, in that region which lies at a distance of between 0 and 10 nanometers from the interface with the ambient environment, such as the air.
The characteristics defined above allow the glazing provided with the substrate according to the invention to fully meet the necessary requirements of the windscreen application, namely the combination of an RL value of less than 7%, and even less than 6%, at normal incidence and less than 10% at an angle of incidence of 60xc2x0, and a TL value of at least 75% at normal incidence.
It is also possible, according to the invention, to choose one of the materials used in the composition of the graded-index xe2x80x9cmonolayerxe2x80x9d so that it provides a hydrophobic function on the external side of the substrate.
The substrate according to the invention is advantageously provided on that face not having the antireflection coating with a stack of thin layers which includes at least one functional layer, for example made a metallic functional layer of the silver type.
As to the type of stack, this may be a stack with the following sequence:
dielectric/silver/dielectric or dielectric/silver/dielectric/silver/dielectric.
For more details concerning these types of stack, reference may be made to European Patent Applications EP-A-0,678,484, EP-A-0,645,352 and EP-A-0,635,528.
It is also possible to use stacks comprising a reflecting and/or filtering layer, such as a nitride layer, for example titanium nitride, as described in Applications EP-A-0,638,527 and EP-A-0,650,938.
The thickness of the metallic functional layer may be between 20 and 25 nanometers so as to give it solar-protection properties.
The preferred sequence of the stack is as follows:
glass/Si3N4/ZnO/Ag/Ti/ZnO/Si3N4/ZnO/Ag/Ti/ZnC/Si3N4. 
One embodiment may consist in depositing this type of stack directly on a flexible transparent substrate, such as one made of polyethylene terephthalate (PET). This may be the product X1R 70(copyright), sold by Southwall.
It is also possible to deposit, on that face of the glazing not having the layer of the invention, a photocatalytic anti-fouling layer based on TiO2, as described in Patents WO 97/10185 and WO 97/10186.
The invention also relates to laminated glazing which includes at least one substrate as defined above.
Such glazing may include tow transparent substrates, especially glass substrates, joined together by a thermoplastic insert comprising at last one sheet of polyvinyl butyral (PVS), that transparent substrate not having the antireflection coating A itself including, on at least one of its faces, an antireflection coating Axe2x80x2, preferably deposited using a magnetron sputtering technique.
This coating Axe2x80x2 preferably has the following sequence:
glass/SnO2/SiO2:Al/Nb2O6/SiO2:Al. 
The dielectric Nb2O6 may advantageously be partially or completely replaced with a material having similar optical properties, such as TiO2 for the purpose of making it easier to deposit the final layer of the stack.
Advantageously, the antireflection coating A is on the 1 face and the antireflection coating Axe2x80x2 is on the 4 face.
Such laminated glazing is perfectly suited to use as a motor-vehicle windscreen. It is also suited to use a glass for protecting photographic images or paintings.
Moreover, it may be noted that the antireflection coating according to the invention may also apply to so-called asymmetrical laminated glazing assemblies, comprising at least one glass substrate and at least one sheet of polymer having energy-absorption properties, such as polyurethane
The choice of the nature of the glass substrate(s) making up the glazing may also prove to be important: the optical and/or thermal properties intrinsic to the glass substrate(s) may be combined with the optical properties of the antireflection coating(s) in order to obtain glazing having overall the desired performance.
Thus, the substrates may be chosen to be made of clear glass, for example such as those sold under the commercial name xe2x80x9cPlaniluxxe2x80x9d by Saint-Gobain Glass. The additional effect of increasing the light transmission of the antireflection coating(s) therefore makes it possible to obtain glazing which is extremely transparent.
However, it is also possible to choose the substrates making up the glazing to be made of glass having reduced energy transmission properties, especially bulk-tinted glass. At the cost of a certain drop in the light transmission, useful solar-protection glazing is obtained, the effect of increasing the light transmission obtained by virtue of the antireflection coating(s) advantageously allowing this reduction in level of transparency to be moderated. Such bulk-tinted glazing, especially suitable for windscreens, is sold, for example, under the name xe2x80x9cSekurisolxe2x80x9d or xe2x80x9cThermocontrolxe2x80x9d by Sekurit Saint-Gobain. Other types of glass having reduced energy transmissions are also useful within the context of the present invention.
These are, especially, bronze-coloured glasses, as described in U.S. Pat. No. 4,190,542 and U.S. Pat. No. 4,101,705, or glasses whose composition has been adjusted more for the purpose of motor-vehicle glazing applications. These are, for example, glasses called TSA+ or TSA++, the content of colouring oxides, of the Fe2O1, FeO and CoO type, of which is adjusted so as to have a selectivity defined by the TL/TE ration of less than 1.30 or even 1.40 to 1.50, and a tint within the greens. For more details, reference may advantageously be made to European Patent Application EP-A-0,616,883. Briefly recalled below is the content of the aforementioned colouring oxides in the glass compositions according to the teaching of that patent (in proportions by weight).
According to a first series:
According to a second series:
The glasses may also be bulk-tinted glasses, especially those tinted in the blue-green, such as those described in Patent Applications EP-A-0,644,164, Wo 95/00828 or WO 96/00394.
All these types of tinted-glass composition may therefore advantageously be chosen so that the glazing has energy transmission values of between 30 and 70%, especially between 35 and 60%, and light transmission values of between 50 and 85%.
Finally, the invention relates to a process for manufacturing a transparent substrate, especially made of glass, provided on at least one of its faces with an antireflection coating A comprising at least one thin layer of thickness e deposited using a radiofrequency or microwave plasma CVD technique using at least two precursors over a time t. According to the invention, at least one deposition parameter is varied continuously over the time t.
The expression xe2x80x9cdeposition parameterxe2x80x9d should be understood to mean, within the context of the invention, one of the following deposition conditions used in the plasma CVD technique: deposition pressure, radiation power, precursor flow rates, substrate temperature, electrical voltage on the substrate.
According to a highly advantageous characteristic, the deposition parameter that is varied over the time t is the flow rate of at least one of the two precursors.
According to another variant of the invention, the antireflection coating of the invention A may also be deposited using another vacuum technique. Preferably, this is sputtering, especially sputtering assisted by a magnetic field. Since oxides are involved, it is possible to choose reactive sputtering with an oxidizing agent in the deposition atmosphere. It is also possible to deposit the coating by CVD not assisted by a plasma, as described, for example, in the French Patent FR 95/08421 of Jul. 12, 1995.